(1) Field of the Invention
The invention relates to apparatus and methods for forming and shaping a workpiece made of thermoformable plastic material. The broad term "thermoformable plastic material" is defined to include materials which contain thermoplastic resins and thermosetting resins. Thermoplastic resins are defined as materials which have the property of softening or fusing when heated and of hardening and becoming rigid again when cooled. Thermosetting resins are defined as materials which have the property of becoming permanently hard and rigid when heated to a critical temperature.
One important type of thermoplastic material is thermoplastic composite material. Thermoplastic composite materials are combinations of a thermoplastic resin and another material differing in composition or form. The constituents retain their physical identities in the composite material, that is, they do not dissolve or otherwise merge completely into each other although they act together. Advanced composite materials are composite materials made by embedding high-strength, high-modulus fibers within an essentially homogeneous matrix. In one particular example, the invention relates to an apparatus and method for forming and shaping aerospace parts made of advanced thermoplastic composite materials. Advanced thermoplastic composites are now being employed as lightweight structural materials in many aerospace uses.
(2) Description of the Related Art
Currently, press forming or shaping of parts made of advanced thermoplastic composite materials for aerospace use is generally accomplished by heating a pre-consolidated sheet or blank of material in an oven to 750.degree.-800.degree. F. and then transferring it by hand to a press as fast as possible where it is formed between dies before it cools. This process is difficult, cumbersome, and time-consuming. In many cases, the blank of material will cool below the melting temperature and begin solidification before the dies can be closed. Forming the part this way will result in damage to the material. Manual handling of the material blanks at elevated temperatures (750.degree.-800.degree. F.) requires thick heat-resistant gloves and this makes accurate positioning of the blank in relation to the dies very difficult.
There is also a process for consolidating flat sheets of advanced thermoplastic composites that is known in the art. The process involves stacking thin layers of flat blanks of thermoplastic composite material. The stack of flat sheets is heated by being placed in a press with hot platens. The hot material is then transferred by hand to another press with cold patens where the material is pressed flat and held under pressure until cooled. Here again the problem is that the material cools rapidly while being moved between presses and, if used to form shaped parts, accurately positioning the hot blanks relative to the dies by hand would be difficult.